Enrile said the country once tried unicameral legislation, which turned out to be inefficient because it lacks check and balance.

Both Enrile and Pimentel said a constitutional convention is still the best way to change the constitution.

"That is the most ideal but it is tedious and will take time. Unless the president will say I'll give you so much time, one year, and if you cannot do it I'll find another avenue to do it," Enrile said.

Pimentel, for his part said, in the event a constituent assembly prevails, public participation must be demanded.

"Public hearings must be conducted and people are given the chance to participate in the hearings conducted," he said.

SLOW DOWN THE CHA-CHA TRAIN

The Duterte administration and allies in Congress have been advocating for a shift to a federal form of government.